At 16, John Walker was a quiet California kid. At 20, he was a Taliban warrior. How did he get from Marin County to Mazar-i-Sharif?

The strangest culture clash of the war in Afghanistan took place on a bright Sunday morning in late November. In the Qala-i-Jangi prison fortress, a few miles west of Mazar-i-Sharif, CIA agent Johnny ("Mike") Spann was sorting through 300 surrendered Taliban soldiers in an attempt to determine which of them were al-Qaeda members. Dressed in blue jeans, with an AK-47 strapped across the back of his black sweater, Spann passed through several rows of Taliban before crouching in front of a prisoner who had been separated from the rest, a mass of tangled hair and tattered clothes once named John Walker....